A lithium ion secondary battery, which uses lithium ion, has a feature that it has high volume/weight energy density as compared with other secondary batteries. Consequently, it is widely used as a power source for consumer equipment such as a mobile phone or a laptop computer. What is expected in the future is that the lithium ion secondary battery will be developed as a power source dedicated to a large-size application such as a electric-motor-driven electric vehicle, which discharges a reduced amount of CO2 and is environment-friendly, and a hybrid vehicle, which is driven by an electric motor and an engine, or a power source for storing renewable energy, such as solar power generation or wind power generation.
To develop a lithium ion secondary battery for use in a large-size application, it is necessary to increase the energy density of the battery. Commonly, various means may be used to increase the energy density of batteries. Such means include, for instance, increasing the volume of active materials, increasing the content ratio of active materials in the electrodes, and increasing the thickness of electrode mixture layer, which is constituted by an active material, a conductive material, and a binder. Simply increasing the thickness of the electrode mixture layer causes a problem that the electrode mixture layer can be readily removed from the current collector and causes the capacity characteristic (hereafter, also referred to as “rate characteristic”) of the lithium ion secondary battery to be decreased. Also, decreasing the particle size of the active material causes such a tendency to appear more noticeably.
To solve at least one of these problems, a technology is proposed as disclosed in PTL 1. This technology includes a current collector constituted by a foil at least one side of which has a roughened surface with an arithmetic average height Ra being within the range of 0.2 to 0.8 μm and a maximum height Rz being within the range of 0.5 to 5 μm, with the amount of an oil component attached to the roughened surface of the foil being in an amount within the range of 50 to 1,000 μg/m2. Use of such a foil will make the electrode mixture layer hardly separable from the current collector due to the roughened surface and in addition use of a suitable amount of the oil component will enable the adhesion of the electrode mixture layer to the current collector to be increased.
In addition, PTL 2 discloses a current collector that includes an aluminum foil having Ra of 0.1 to 10 μm and PTL 3 discloses a current collector that includes an aluminum foil having Ra of 2.5 or more.